The effect of solutes, water activity (a w , 0.890-0.980) and temperature (5-25 8C) on the mycelial growth rate of Penicillium expansum was evaluated. The growth rate dropped as the temperature and a w of the medium decreased. NaCl was the solute causing the greatest growth rate reduction, followed by glucose, glycerol and sorbitol. Statistical analysis of the results showed a significant effect of solute, a w , temperature and combinations of two or three of these factors ( P b 0.0001). Whatever the solutes and a w values, the initiation of colony growth required an additional day at 15 8C and 5 8C as compared to initiation at 25 8C. Growth models based on the results obtained with sorbitol and glycerol differed only slightly, with R 2 values of 97.00% and 97.95%, respectively. The response surfaces of both quadratic polynomial models showed that P. expansum should be able to grow at low a w (0.890) and that growth at 25 8C should be fastest at a w values ranging from 0.960 to 0.980. Both models presented a good fit between predicted and observed values. D

The objective of this work was to develop validated models predicting the 'in vitro' effect of a w and temperature on the radial growth of Botrytis cinerea. The growth rate (g, mm d− 1 ) of B. cinerea was calculated at three incubation temperatures (25°C, 15°C, 5°C) and six water activities (ranging from 0.995 to 0.890). The water activity was adjusted with glucose, NaCl, glycerol, or sorbitol. Statistical analysis showed a significant effect of temperature, solute, a w , and their two-and three-way interactions on the growth rate. No growth was observed at a w = 0.93 in the presence of NaCl or at 0.89 in the presence of a non-ionic solute. The maximum colony growth rate decreased when the incubation temperature and water activity was lowered. Secondary models, relating the colony growth rate with a w or a w and temperature were developed. Optimum a w values for growth ranged from 0.981 to 0.987 in glycerol-, sorbitol-, or glucose-modified medium and were close to 1 in NaClmodified medium. A quadratic polynomial equation was used to describe the combined effects of temperature and a w on g (mm d − 1 ) in the presence of each solute. The highest and lowest radial growth rates were observed in models based on glucose and NaCl respectively, whatever the incubation temperature. All models prove to be good predictors of the growth rates of B. cinerea within the limits of experiments. The quadratic polynomial equation has bias factors of 0.957, 1.036, 0.950, and 0.860 and accuracy factors of 1.089, 1.070, 1.120 and 1.260 in media supplemented with glucose, NaCl, glycerol and sorbitol respectively. The results from modelling confirm the general finding that a w has a greater influence on fungal growth than temperature.

The objective of this work was to develop models predicting the combined effects of relative humidity (RH, 75-98%), temperature (5-25°C), and initial applied yeast concentration (10 4 -10 8 CFU/ml) on the apple-surface population densities of two biocontrol agents fused against postharvest diseases; the antagonistic yeasts Pichia anomala strain K and Candida oleophila strain O. Experiments were carried out according to a Box-Behnken matrix. Multiple regression analyses showed that both models yielded a good prediction of yeast density. The effect of relative humidity appeared greater than that of temperature. The number of yeast colony-forming units per square centimeter of apple fruit surface increased with increasing relative humidity, temperature, and initial applied yeast concentration. The models predict that under optimal growth conditions (25°C, 98%), strains O and K should reach a density of 10 4 CFU/cm 2 when applied initially at 2 × 10 7 (strain O) or 10 7 CFU/ml (strain K). The model results suggest that rainfall was likely the principal cause of the variability of yeast efficacy reported for previous preharvest orchard trials spanning two successive years. Temperature may also contribute to this variation. The models developed here are important tools for predicting population densities of both strains on the apple surface within the experimental limits. The use of these results should contribute to achieving yeast densities of 10 4 CFU/cm 2 on apples by controlling yeast application and environmental factors such as relative humidity and temperature. The results of this study also confirm our previous in vitro findings that water activity has a greater effect than temperature on yeast population density.

In vitro effects of water activity, temperature and solutes on the growth rate of P. italicum Wehmer and P. digitatum Sacc. IntroductionCitrus fruits are generally stored in temperatures ranging from 0 to 7°C and relative humidities from 85% to 90%. During their storage, fruits are affected by several decays (Brown and Miller 1999). Penicillium digitatum Wehmer and P. italicum Sacc. are the main postharvest pathogens of citrus fruits, responsible, respectively, for green and blue decay. In turn massive spore production by these two pathogens ensures their presence wherever fruit is produced, handled and stored. Contamination takes place only through wounds where nutriments are available to AbstractAims: To evaluate the effect of water activity (a w 0AE98-0AE89, adjusted with glycerol, sorbitol, glucose, or NaCl) and temperature (5-25°C) on the lag phase and radial growth rate (mm day )1 ) of the important citrus spoilage fungi, such as Penicillium italicum and Penicillium digitatum grown in potato dextrose agar (PDA) medium. To select, among models based on the use of different solutes, a model fitting accurately the growth of these species in relation to a w and temperature. Methods and Results: Extensive data analyses showed for both Penicillium species a highly significant effect of a w , temperature, solutes and their interactions on radial growth rate (P < 0AE0001). Radial growth rate was inhibited and the lag phase (i.e. the time required for growth) lengthened as the a w of the medium decreased. NaCl appeared to causes the greatest stress on growth when compared with other nonionic solutes. Penicillium italicum stopped growing at 0AE96 a w and P. digitatum at 0AE93 a w . Under the dry conditions where growth was observed, P. italicum grew faster than P. digitatum at low temperature and P. digitatum remained more active at ambient temperature. Multiple regression analysis applied to the square roots of the growth rates observed in the presence of each solute showed that both the 'glycerol model' and the 'sorbitol model' yielded a good prediction of P. italicum growth and the 'sorbitol model' gave an accurate fit for P. digitatum growth, offering high-quality prediction within the experimental limits described. Conclusions: Mathematical models describing and predicting, as a function of a w and temperature, the square root of the radial growth rate of the agents responsible for blue and green decays are important tools for understanding the behaviour of these fungi under natural conditions and for predicting citrus fruit spoilage. Significance and Impact of the Study: Implementation of these results should contribute towards a more rational control strategy against citrus spoilage fungi. spore germination. Fruit decay thus begins at the site of injury. Blue decay develops less rapidly than green decay under ambient conditions. Green decay is often observed in mixed infections. Blue decay is more common in fruit held in cold storage during the summer; it can spread through packed cartons more easily than ...

The yeast Pichia anomala strain K was selected in Belgium from the apple surface for its antagonistic activity against post-harvest diseases of apples. The efficacy of this strain against P. expansum was evaluated in the laboratory in three scenarios designed to mimic practical conditions, with different periods of incubation between biological treatment, wounding of fruit surface, and pathogen inoculation. Higher protection levels and higher final yeast densities were obtained when the applied initial concentration was 1×10 8 cfu ml −1 than when it was only 1×10 5 cfu ml −1 . The protection level correlated positively with the yeast density determined in wounds and was influenced by apple surface wetness. In orchard trials spanning two successive years, biological treatment against P. expansum, based on a powder of P. anomala strain K (1×10 7 cfu ml −1 ), β-1,3-glucans (YGT 2 g l −1 ), and CaCl 2 .2H 2 0 (20 g l −1 ), was applied to apples pre-or post-harvest under practical conditions and its effect compared with standard chemical treatments. The first year, the highest reduction (95.2%) against blue decay was obtained by means of four successive fungicide treatments and the next-highest level (87.6%) with pre-harvest high-volume spraying of the threecomponent mixture 12 days before harvest. The second year, the best results were obtained with post-harvest Sumico (carbendazim 25% and diethofencarb 25%) treatment and post-harvest biological treatment, both by dipping the apples, 88.3 and 56.3% respectively. A density threshold of 1×10 4 cfu cm −2 of strain K on the apple surface seemed to be required just after harvest for high protective activity, whatever the method and time of application. In the case of pre-harvest biological treatments, variations in meteorological conditions between the 2 years may have considerably affected strain K population density and its efficacies.